Catalytic mid-barrel hydrocracking is a petroleum refining process of increasing importance due to the increased demand for mid distillate fuels. Forecasts of consumer demand for refined products are predicting shifts from the gasoline range hydrocarbon fuels to heavier, higher boiling mixtures such as diesel, turbine and heavy diesel. In general, the process consists of converting heavy petroleum feedstocks boiling above 700.degree. F., such as vacuum gas oil and residual feedstocks, to a lower boiling range. The favored product boiling range is 300.degree.-700.degree. F. turbine and diesel fuels. The catalysts used in these hydrocracking processes are dual functional types, consisting of a hydrogenation component such as a Group VIII noble metal or a combination of Group VIII (Ni,Co) and Group VIB (Mo, W) metals, in combination with a solid acid catalyst, such as the LZ-10 zeolite, amorphous silica-alumina gel, transition aluminas or aluminosilicates. The latter components act as acidic cracking catalysts and they may also act as support for the metal components.
Of the solid acid components, it is qenerally understood that aluminosilicate zeolites are the most active in the sense that they convert the highest fraction of feedstock to lower boiling products under comparable operating conditions. Activity however, is only one of three essential properties of a mid-barrel hydrocracking catalyst. The other two properties are selectivity to produce the desired products (i.e. turbine and/or diesel) exclusively, and stability which is a measure of the useful operating life of the catalyst. It has been found that the high activity of strong acid zeolite catalysts does not compensate for their poor selectivity for turbine and diesel oil, and accordingly no commercial mid-barrel catalyst utilizes strong acid zeolites as the principal acid cracking component. Instead this function is provided either by amorphous compositions such as silica-aluminas derived from silica-alumina gels or by the mild acid LZ-10 zeolite, UHP Y zeolite described in U.S. Pat. No. 4,401,556, which shows much higher selectivity and lower activity than strong acid zeolites.
The chemistry of gasoline and mid-barrel hydrocracking processes are significantly different. In gasoline hydrocracking multiple fragmentation of the feed molecules is required. In mid barrel hydrocracking on the other hand, the average feedstock molecule should be split only once and very near the center of the molecule in order to maximize the mid-barrel fraction, and thereby minimize the production of light hydrocarbons, such as C.sub.1 -C.sub.4 and gasoline. The zeolite component of catalysts employed for gasoline hydrocracking are strong-acid zeolites, such as Y-82, ReY or LZ-210 zeolite. In the production of diesel and turbine fuels multiple chain branching and multiple cracking are undesirable, consequently, weak or mild acid catalysts are required which yield considerably less isomerization and less multiple fragmentation.
The principal prior art of which the applicants are aware is GB 2114594A, which was published on August 24, 1983, to Bradrick, which describes the use of acid leaching of Y, ultrastable Y, X, decationized Y, and their derivatives. The steaming causes substantial loss in crystallinity and forms cracks and fissures in the crystal. Aluminum is removed from the Y-zeolite framework lattice during this treatment. Consequently, the resulting dealuminated crystal framework lattice has a higher SiO.sub.2 /Al.sub.2 O.sub.3 ratio and consequently a smaller crystal cell constant. The acid leaching step causes further substantial reduction in the lattice constance (a.sub.o) as well as substantial reduction in zeolite crystallinity.
The starting material for the preparation of the catalyst base for the present invention is the LZ-10 (UHP-Y) decribed in U.S. Pat. No. 4,401,556. This Y product is prepared from ammonium exchanged Y by steaming, followed by a further ammonium exchange to remove residual soda and resteaming to achieve the requisite UHP-Y characteristics of having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 4.5 to 35, the essential X-ray powder diffraction pattern of zeolite Y, an ion exchange capacity of not greater than 0.070, a unit cell dimension a.sub.o of from 24.20 to 24.45 A, a surface area of at least 350m.sup.2 /g(B-E-T), a sorptive capacity for water vapor at 25.degree. C. and a p/p.degree. value of 0.10 of less than 5.00 weight percent and a Residual Butanol Test value of not more than 0.40 weight percent.
In the treatment of the UHP-Y product in accord with the present invention by acid or other extracting agents to remove amorphous occluded debris formed during previous treatments, there is no significant change in lattice unit cell dimension, a.sub.o, while there is a substantial increase in Y zeolite crystallinity and surface area in the treated product.
The dramatic difference in the x-ray crystallinity of the acid extracted product of this invention compared to the material prepared according to the teachings of Bradrick (GB 2114594A) is illustrated in FIG. 1. Whereas the preparation of acid extracted zeolites by the Bradrick method leads to products having an x-ray crystallinity significantly less than the starting material, the preparation of such acid extracted products according to the present invention leads to products having x-ray crystallinity significantly greater than the starting material over a wide range of SiO.sub.2 /Al.sub.2 O.sub.3 ratios.
Under the conditions described in GB2114594A, the mid-barrel selectivity is in the range of 40 to 50% conversion per pass for products boiling below 700.degree. F. Selectivity can be increased to levels in excess of 60% mid-distillate for products in boiling range 400.degree.-700.degree. F. at 50% conversion per pass.
In the process of the present invention, selectivity to mid-barrel products distilling in the boiling range of 300.degree.-700.degree. F. is in the range of from about 85 to 90% at feed conversion of 60% per pass.
GB 2114594A refers to SiO.sub.2 /Al.sub.2 O.sub.3 ratios greater than 10, and gives data for materials with ratios as high as 100. In the procedure of the present invention much lower ratios are effective. SiO.sub.2 /Al.sub.2 O.sub.3 ratios of 8 to 12 are shown to be more effective to produce mid-barrel products using the present procedures for synthesizing the catalyst than the much higher ratios described in the reference.
The reference describes a process in which the a.sub.o of the treated Y zeolite decreases in a regular fashion with increasing HCl/zeolite concentration (See FIG. 2 thereof). In the presently described process there is very little, or no, change in the cell constant with acid concentration or number of acid treatments.